As it relates to an American returning from abroad, it doesn't appear what "briefly" means. From about 5 minutes of Googling, it appears anywhere up to 12 hours of detention is possible (based on a 2008 CBP internal memo).

Detentions have lasted more than 24 hours though, sometimes for days:

"Median Number of Hours Individuals Were Held in Detention, by Sector"

https://i.imgur.com/MNYTJzF.png